Nonagram

The Nonagram (9 Point Star) is the symbol that Slipknot has drawn by themselves. They didn't borrow it from any existing image.

The Nonagram, for the band, is a symbol of unity with each other, it's the symbol of 9 people. 9 points - 9 people. Each point represents each band member.
The 9 Point Star can be seen on their Vol. 3 Jumpsuits (the Nonagram is located on their right sleeve). And no, the Nonagram is not a satanic symbol. It does not represent Satan or evil, it represents the battle with the fake world, Slipknot's battle.
One of the band members that has a Nonagram tattoo is Chris Fehn (#3).
Well, as you should all know, Slipknot are all from Des Moines, IA. And the words "Des" and "Moines" have 9 letters when put together. So that tells you something right just there.

It is composed of three equilateral triangles, each rotated 0, 40 and 80 degrees.